1. Field of the Invention
This invention relates generally to shaft seal devices. This invention relates especially but not exclusively to shaft seal devices for use with a stirrer or the like providing for a seal of a stirred fluid under a high pressure or extremely high pressure.
2. Description of the Related Art
A shaft seal device as a relative art of the present invention is illustrated in FIG. 4. FIG. 4 is a half cross-sectional view of a high pressure mechanical seal device.
In FIG. 4, a rotary shaft 175 is supported in a rotary movable manner by a bearing portion which is disposed in a housing 151 of the stirrer 150. The rotary shaft 175 extends throughout the inside and the outside of the housing 151. The inside of the stirrer 150 is filled with a high pressure sealing fluid A.
The housing 151 through which the rotary shaft 175 extends is divided into three chambers 152A, 152B and 152C. The chamber 152 comprises, counted from the sealing fluid A side, a first chamber 152A, a second chamber 152B and a third chamber 152C.
When the pressure of the sealing fluid A is 20 MPa say, a pressure fluid of 15 MPa which is a little lower than that of the fluid A is fed to the first chamber 152A from a first pipe 153A. Similarly, a pressure fluid of 10 MPa which is further lower than the pressure of the first chamber 152A is supplied to the second chamber 152B from a second pipe 153B. Likewise, a pressure fluid of 5 MPa which is yet lower than the pressure of the second chamber 152B is supplied to the third chamber 152C from a third pipe 153C. Therefore, a plurality of chambers are necessarily disposed in such a manner that the high pressure of the sealing fluid A is decreased step by step.
Without such an elaborate means, a high pressure may cause a damage to the first mechanical seal 110.
The first chamber 152A, second chamber 152B and third chamber 152C contain respective mechanical seal devices 110, 120, 130 within, and these mechanical seal devices 110, 120, 130 are, respectively, referred to, from the sealing fluid A side towards the atmospheric region, a first mechanical seal 110, a second mechanical seal 120 and a third mechanical seal 130.
The first mechanical seal 110 is a high pressure mechanical seal due to a high pressure nature of the sealing fluid A. Likewise, high pressure mechanical seals are used for the third mechanical seal 120 as well as the second mechanical seal 130 in accordance with a level of the pressure to which they are subjected.
Sealing fluid A used in a stirrer 150 is a high pressure, and mechanical seal devices 110, 120, 130 are typically employed because elastic rubber seals cannot sustain such a high pressure.
These mechanical seals 110, 120, 130 are specifically provided for a high pressure use, and they are mounted in such a way that a shaft sleeve 111 is fitted over the rotary shaft 175 via an O-ring. A spring support 112 engages the sleeve 111 at the shoulder part of the sleeve 111, and a rotary seal ring 113 is fitted to the other end of the sleeve 111 in a movable manner via an O-ring. A biasing spring 114 supported by the spring support 112 is disposed to exert an urging force to the rotary seal ring 113.
On the other hand, a stationary seal ring 115 is fitted to a retaining portion of the housing 151 via an O-ring which has an opposing seal face 155A making a sealing contact with a slidable seal face 113A of the rotary seal ring 113.
The second mechanical seal 120 and the third mechanical seal 130 are constructed in a similar manner to the way that the first mechanical seal 110 is constructed.
The first mechanical seal 110 effectively seals the high pressure sealing fluid A within the stirrer in cooperation with the fluid pressure within the first chamber 152A which is a little lower than the pressure of the fluid A.
Likewise for the second mechanical seal 120, it seals the pressure fluid within the first chamber 152A in cooperation with the fluid pressure within the second chamber 152B which is two-step lower than the pressure of the fluid A. Similarly, the third mechanical seal 130, it seals the pressure fluid within the first chamber 152B in cooperation with the fluid pressure within the second chamber 152C which is three-step lower than the pressure of the fluid A.
Use of such high performance mechanical seals, which reduce the fluid pressure in a stepwise manner from chamber to chamber, is essential to provide a secure seal for the sealing fluid A under a high pressure. In other words, when sealing a high pressure fluid, without gradually reducing the high pressure in multiple steps, a single mechanical seal device cannot sustain such a high fluid pressure and will be worn out or damaged at its early stage.
A number of expensive mechanical seals are needed in order to realize such a mechanical seal device in multiple steps and this makes a sealing construction larger and longer. In addition, if one of the multiple mechanical seals is in failure, a pressure differential between the two adjacent mechanical seals becomes large and the increased pressure differential causes other individual mechanical seals to fail. Also a mechanical seal arranged in such a multiple step structure imposes difficulties in machining for mounting the mechanical seals as well as an assembly of the mechanical seal device. This increases a fabrication cost.
Furthermore, providing a fluid of distinct pressures to individual chambers so that the pressure is varied in multiple steps requires a cost increase due to pumps and their related controllers. A modulation of the fluid pressure is also difficult.
The present invention is introduced to resolve the above mentioned problems. A primary technical goal which this invention tries to achieve is to reduce the number of mechanical seals for use and to provide for a seal of a high pressure fluid by utilizing inexpensive mechanical seals while avoiding a total cost increase of a shaft seal device which is caused by employing a plurality of expensive mechanical seals.
Sealing a high pressure fluid by means of a plurality of mechanical seal devices will lead to a large construction because of a disposition of many chambers for retaining the seals. Therefore, another goal of the present invention is to reduce the size of the shaft seal device by making the installation space compact.
Yet another goal for the present invention is to provide a shaft seal device with enhanced pressure proof and seal performance against a high pressure fluid as well as failure prevention induced by the fluid pressure.